1. Field of the Invention
This invention relates to the process for making shell investment molds for the casting and solidification of superalloys therein.
2. Description of the Prior Art
Shell investment molds are employed to produce castings of a wide variety of alloys with a refractory material, compatable with the alloying being cast forming the inner mold wall. The secondary or back-up coats usually are composed of a high alumina-silicate refractory of an appropriate grain size to insure production of a useable mold. Long periods of time, up to 10 hours, coupled with high temperatures (1500.degree.-1600.degree. C) caused undesirable mold defects, such as total collapse after casting the alloys, premature cracking and mold warpage. All of these, of course, produce undesirable and unacceptable castings. Mold-metal reactions, such as "pock marks", were also noted, suggesting that the inner or primary coats became contaminated with excessive amounts of SiO.sub.2, Na.sub.2 O, and other fluxing agents which were not compatable with the alloy being cast.
An object of this invention is to provide a new and improved process for making shell investment molds for the casting and solidification of superalloys which overcome the deficiencies of the prior art.
An object of this invention is to provide a new and improved process for making a slurry suitable for making shell investment molds suitable for use for extended periods of time at high temperatures.
Another object of this invention is to provide a new and improved primary slurry composition for an investment mold, the material of which at the mold-metal interface is non-reactive to the metal in contact therewith.
Another object of this invention is to provide a new and improved alumina slurry to withstand the effects of mold-metal reactions at the mold-metal interface, such as required for the directional solidification of nickel-base superalloys and high-temperature nickel-base eutectic alloys requiring long solidification periods to obtain the desired cast structure.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.